1. Field of the Invention
This application relates to an electrical circuit and, in particular, to a power supply circuit for operating a light source, particularly, an array of light emitting diodes (LEDs).
2. Discussion of the Art
Incandescent lamps are used in a wide variety of environments and generally have found substantial commercial success in meeting various demands. More recently, however, industry is seeking an alternative light source that is more efficient, has an extended life, and can endure the rigors of applications that experience constant exposure to the elements and frequent on/off switching. An exemplary use of an incandescent lamp in this type of environment is in traffic signals. While incandescent lamps have traditionally been used in traffic signals, the incandescent lamp encounters constant exposure to the elements and has a relatively short life span of typically around eight thousand hours, which is further shortened if it is frequently switched on and off. Additionally, the incandescent lamp is inefficient due to the fact that much of the light energy produced by it is wasted by filtering the light. As a result, traffic signals utilizing incandescent lamps require frequent maintenance and typically must be replaced once or twice a year.
Light emitting diodes (LEDs) have been suggested as alternative light sources due to their robust structure that are able to withstand constant exposure to the elements and the long life associated with their efficient operation. Known advantages of using an LED array in lieu of an incandescent lamp include increased efficiency, little or no maintenance, greater resistance to the elements, and greater mechanical durability.
Additionally, an LED array consumes less power to produce the same light output as an incandescent lamp. Further advantages are that an LED array may function for more than twenty years before requiring replacement, an LED array does not require a light reflector, and a fault in the LED array does not necessarily mean that the entire LED array will fail. In addition to performing the same functions as an incandescent lamp, a single LED array may also be used to display different illuminated symbols such as xe2x80x9cno left turnxe2x80x9d, xe2x80x9cturn onlyxe2x80x9d, and xe2x80x9cdo not enterxe2x80x9d.
Despite all of these advantages, however, there are still several concerns which have prevented widespread adoption of LED arrays in, for example, traffic signals. The most significant is that an LED array is not easily retrofitted. In the environment of traffic signals, incandescent lamps typically operate with a 120 volt 60 Hz AC power supply, and LEDs typically require a DC current of approximately 5 to 20 milliamps and a forward operating voltage of between 1.5 to 2.5 volts. Second, xe2x80x9cstandardxe2x80x9d incandescent lamp traffic signal housings are designed to accept a xe2x80x9cstandardxe2x80x9d incandescent bulb.
While these issues have been addressed in the prior art, the known solutions have raised other issues. For example, retrofitting an existing incandescent lamp traffic signal using only an inductor or L-C circuit connected to LED pairs connected in series presents a significant problem since the required inductor for an array of approximately twenty LEDs is 6 Henries. An inductor of this size is very heavy, making the LED assembly much heavier than a traditional incandescent lamp assembly. Thus, a pole or electrical line to which a traffic signal is mounted must be sufficient to support the increased weight for the long duration. Accommodations for an enlarged inductor must also be made in the traffic signal housing, along with attendant difficulties regarding installing, maintaining, or retrofitting the LED array in the limited space of a standard incandescent lamp traffic signal. Needs of a technician working in a bucket ladder high above the ground must also be considered in such a design.
Accordingly, a need exists for an alternative manner of supplying power to LED arrays, particularly those used in traffic signals.
The present invention provides a more cost efficient electrical circuit for supplying power to an LED array.
A power supply circuit for an LED array includes a rectifier for converting an AC current to DC current; a starting circuit coupled to the rectifier for providing a path for the AC current; a gate drive arrangement coupled to the starting circuit; a resonant converter circuit regeneratively controlled by the gate drive arrangement; and a resonant load circuit coupled to the resonant converter circuit for inducing the AC current in the resonant load circuit. The resonant load circuit incorporates a resonant inductance, a resonant capacitance coupled to the resonant inductance, and a load connected in parallel to the resonant capacitance.
A plurality of light emitting elements and a capacitor define at least a portion of the load.
In other preferred embodiments, the load includes at least one pair of oppositely polarized light emitting elements connected in parallel or at least one pair of oppositely polarized branches of light emitting elements.
This circuit has a number of advantages over the prior art. The power supply circuit uses smaller components and decreases the space requirements in the electrical compartment of a traffic signal than any known prior art circuit.
Another advantage resides in the use of integrated circuits which weigh less and provide for easier placement in traffic signals.
Still another advantage is realized since integrated circuits will fit on the same circuit board as the LED array.